Congestion-based network traffic policing is a promising network resource control paradigm that accounts for user traffic in the event of network congestion. It has been argued, for example in B. Briscoe, “Flow Rate Fairness: Dismantling a Religion”, ACM Computer Communications Review, 37(2), 63-74 (April 2007), that flow rate fairness, which has been used in the past, is not a reasonable mechanism for resource allocation and accountability of network resources. Instead, it is suggested that a cost-based mechanism would provide a better resource allocation paradigm, in which “cost” means the degree to which each user's transfers restrict other transfers, given the available network resources. The metric that has been suggested for measuring this cost is the volume of network congestion caused by each user. A network traffic policing mechanism based on congestion offers a net-neutral way for network operators to manage traffic on their networks.
There have been several proposals for implementing congestion-based network traffic policing. For example, Re-ECN is a proposal that has been made in the Internet Engineering Task Force (IETF) Congestion Exposure (CONEX) Working Group. As will be discussed in greater detail below, Re-ECN, or re-feedback of explicit congestion notification provides a feedback mechanism through which packets expose the congestion that they expect to cause. Unfortunately, implementing Re-ECN would require protocol extensions both at end hosts and at the IP level. In order to deploy such protocol extensions, numerous vendors of network equipment and software would need to support such extensions, which makes wide scale commercial deployment of these technologies unlikely, at least in the near future.
Another congestion-based policing proposal has been put forward by Comcast. This proposal is detailed in IETF RFC 6057: Comcast's Protocol-Agnostic Congestion Management System, which can be found at http://tools.ietf.org/html/rfc6057. The Comcast proposal provides a mechanism for congestion management that is protocol agnostic and that requires no modifications to the communication stack. The system monitors users' network usage, and dynamically decides when congestion has occurred and which user traffic should be throttled. Unfortunately, the system proposed by Comcast seems to have a number of drawbacks, as will be discussed in greater detail below. First, while it is mentioned that the system may be applicable to other types of networks, as described, the system is relatively specific to the broadband service architecture used by Comcast. Also, the Comcast proposal attempts to determine congestion based on a volume at a customer or line level, rather than looking at the “cost”—i.e. congestion volume—on a flow level.